“Supernatural” and “X-Files” are both long-lived TV shows that follow two protagonists as they take on the paranormal. But the similarities between the two shows go even deeper than that.

Here are five episodes that overlap in plot, monster type or style.

1. “Monday” / “Mystery Spot”

One interesting plot device shared by “X-Files” and “Supernatural” is the concept of the time loop. Perhaps popularized by movies like “Groundhog Day” or, more recently, TV shows like Netflix’s “Russian Doll,” this trope puts characters in a loop where they relive the same day over and over. While often this is the basis of the plot, forming both the overarching conflict and resolution for the stories, “X-Files” and “Supernatural” both find ways to work this type of storytelling into their regular narrative structure.

“Mystery Spot” is a popular episode of “Supernatural” that puts Sam in a time loop where he’s forced to live through the death of his brother Dean repeatedly. As with the “Groundhog Day” standard, Sam is aware that he’s in a time loop and the show focuses on how he tries to break his way out, trying everything in his arsenal to prevent his brother’s death. In general, this show’s take on the time loop trope is very by the book.

Its departure from tradition begins after Sam confronts the god-like entity responsible for putting him in this time loop. Sam seemingly then breaks out of his loop and makes it to the next day, only for his brother to die once again. This time Sam lives past his brother’s death and the show presents a montage of the next sixth months as Sam tries to ineffectually cope with this traumatic event.

During these months, he is seen remorselessly searching for the being responsible and killing everything in his way. Eventually, it’s shown that this extended timeline was also a trick, and Sam wakes up six months earlier with his brother still alive.

“Monday” focuses on Mulder as he experiences the same day over and over, yet unlike the protagonist in “Groundhog Day,” or in the “Supernatural” episode, he’s unaware of being stuck in a time loop. As Mulder’s day unfolds he runs into a series of accidents, from his waterbed springing a leak to accidentally tearing his paycheck in half, that eventually lead him to a bank in the middle of a robbery. The crime goes wrong and in the process, over and over again, Mulder is shot. His time loop ends with his death, which he doesn’t remember the next day.

Interestingly, “X-Files” expands the canon of time loops with its treatment. Mulder starts out the episode without the ability to know that he’s repeating the same day. But with each repetition he becomes more aware of his predicament, even finding ways to send himself messages between repetitions.

And while Mulder is unaware of the time loop, a new side character is presented who does understand what is happening. In fact, instead of confronting the cause of the time loop like in “Supernatural,” or learning some sort of moral lesson as in “Groundhog Day,” the time loop in “X Files” ends when this character sacrifices herself as the victim of the robbery-gone-wrong.

I’m initially struck by how differently these shows treat the same “Groundhog Day” trope — “Supernatural” plays it by the book while “X-Files” asks the audience to imagine what it would be like if the main characters were caught in someone else’s time loop. “Supernatural” treats it with humor, while “X-Files” plays it for the drama. “Supernatural” ties it into established characters and even finds a way to relate it to the main storyline, while “X-Files” gets the audience to feel for characters it’s never met before and will not meet again.

Yet, despite these differences, and the depiction of two very different takes on the same theme, both episodes end on a serious, traumatic note. “Supernatural” throws one of its main characters into six months of grieving and killing, six months that Sam will not get back when the time loop resets, leaving him with dreadful memories. “X-Files” asks an innocent bystander, not responsible for the bank robbery or Mulder’s death, to intervene and break the time loop with her death.

2. “Hollywood A.D.” / “The Monster at the End of This Book”

On a lighter note, another repeated theme that appears in both TV shows is the meta-narrative, where each show appears as media within itself. In “X-Files,” a movie loosely based on the exploits of Mulder and Scully depicts the protagonists, and even the idea of the supernatural, as caricatures. The episode, “Hollywood A.D.,” shows a Hollywood writer following around Mulder and Scully to get ideas for the movie, and even consulting them on casting choices. The overarching tone is lighthearted, taking jabs at the movie industry while allowing the audience to laugh at the ridiculousness of the movie-within-a-show.

The “Supernatural” episode, “The Monster at the End of This Book,” takes a more serious approach. Sam and Dean are confronted with a series of books titled “Supernatural” that seem to describe all of their past adventures and even predict their future. They soon find out that these books seem prescient because they’re written by a prophet, who has actual knowledge of the future. The episode follows the boys as they try to escape their pre-written future, with events that tie into the larger storyline of the season.

The two episodes differ in overall tone, with the meta-narrative functioning as a point of humor in “X-Files” but a cause of strife in “Supernatural.” But the presence of each show as a fiction within itself is a unique narrative device that immediately associates these episodes. Perhaps such weirdness is easy to place in TV shows that otherwise are based on strange occurrences — audiences of “X-Files” and “Supernatural” won’t question it.

3. “X-Cops” / “Ghostfacers”

Perhaps one of my favorite examples of a shared hook is embedding the main storyline into a fake episode of reality TV. The “X-Files” episode, “X-Cops,” tells the story of the search for a monster that takes the form of your worst fear, but embeds it within the format of an episode of “Cops.” The shaky, handheld cameras are following around a group of cops who run into Mulder and Scully after being called to investigate a disturbance in a neighborhood.

The “Ghostfacers” episode of Supernatural has a similar gimmick. The episode follows around a group of young ghost hunters as they prepare to spend the night in a haunted house, only to have their investigation interrupted by Sam and Dean.

Coincidentally, both episodes follow an all-night investigation, where the horror ends because the sun comes up. In “X-Files,” the creature cannot keep attacking after sunrise, and in “Supernatural,” the doors to the house they’re stuck in unlock at sunrise too. More mechanically, both episodes also feature a different theme song than the shows usually play, including the iconic “Cops” theme song, “Bad Boys.”

These reality TV episodes introduce new filming techniques and center characters other than the standard protagonists, making for interesting TV. However, despite the general similarities, “X-Files” and “Supernatural” handle the format differently.

The cops in “X-Files” are reluctant to believe in the monster that some of them have seen, which is in keeping with the general “X-Files” tradition of combatting skepticism. The Ghostfacers in “Supernatural” believe too readily in ghosts — it’s what led them to the haunted house in the first place.

Mulder and Scully participate in the “Cops”-esque TV show because they are instructed that the FBI has nothing to hide. Sam and Dean, however, participate only because they have to in order to stay alive. Once they make it out of the house, they erase the videos so that no one else will see.

Additionally, the ghosts in the “Supernatural” haunted house are shown throughout the videos — they clearly exist and interact with the characters. However, in “X-Files” the monster manages to avoid being filmed. For a monster that supposedly takes a different form for each person, its absence from the incontrovertible video record is notable.

4. “Arcadia” / ”Bugs”

“X-Files” and “Supernatural” both take a brief sojourn into suburban life. One episode from each series follows the protagonists as they journey into picturesque housing developments that are hiding supernatural secrets. Mulder and Scully pose as a couple in “Arcadia,” providing the atmosphere of comic relief that juxtaposes nicely with the serious, Stepford-esque plotline. “Bugs” finds Sam and Dean squatting in an unsold house in a new development, repeatedly mistaken for a gay couple, which they play for humor.

Both episodes deal with monsters that come from the ground — a Tulpa created from the garbage on which the development was built in “Arcadia” and a series of swarming bugs in “Bugs.” In both cases, the development owner is at fault for the monster infestation, though “Supernatural” goes one step further in resorting to the clichéd Native American curse as the origin.

These episodes are so similar, in fact, that their biggest difference may just be their reception. “Bugs” is generally reviled and is constantly listed as one of the worst episodes of “Supernatural” to ever air. “Arcadia,” on the other hand, is often listed among the funniest “X-Files” episodes with one of the best monsters of the week. Why the formula works for Mulder and Scully but not Sam and Dean is not immediately clear, but certainly indicates that no matter how similar the shows can seem, they are indeed very different.

5. “Shapes” / “Wendigo”

Any television show that deals with the paranormal will inevitably draw from the same pool of monsters. It’s no surprise that there are some episodes of both “The X-Files” and “Supernatural” that confront the same cryptozoological villains. Both shows feature an episode about the werewolf-esque monster called the Wendigo — “Shapes” in “X-Files” and just the straightforwardly named “Wendigo” in “Supernatural.”

But this section could just as easily have been about the Tulpa in “Arcadia” and “Hell House,” or the Jersey Devil in “The Jersey Devil” and “How To Win Friends And Influence Monsters,” or any number of both “X-Files” and “Supernatural” episodes that feature ghosts or vampires. Both shows confront many of the same villains drawn from a shared folk lore.

However, these shared monsters almost always appear in monster-of-the-week episodes, stories that use the established characters in narratives that resolve in one episode. The overarching, seasonal storylines of each show tend to focus on very different aspects of the paranormal — aliens for “X-Files” and angels and demons for “Supernatural.” The forays into American folklore are here for one episode and then easily dismissed.

The fact that there are some shared narrative styles, gimmicks and monsters between “X-Files” and “Supernatural” is not necessarily surprising — both shows deal with the paranormal and have many seasons and hundreds of episodes to fill with stories. Yet despite the number of similar episodes, each show treats their narratives with a unique style and to a unique end and are therefore absolutely worth watching. In fact, the pairs of episodes I’ve listed make for great double-features — enough shared content to relate them but enough differences to make them interesting.